Principal results

Integrated Ocean Drilling Program (IODP) Site U1309 is located on the central dome of Atlantis Massif, 14–15 km west of the median valley axis of the Mid-Atlantic Ridge (MAR), where the seafloor coincides with what is interpreted to be a gently sloping, corrugated detachment fault surface (Figs. F1,F2). Two drill holes at this site (Holes U1309B and U1309D) penetrate a multiply intruded and faulted crustal section, providing core that documents the interplay between magmatism, deformation, and hydrothermal alteration prior to, during, and subsequent to a period of footwall displacement and denudation associated with detachment faulting. Five shallow-penetration holes (Holes U1309A and U1309E–U1309H) were designed to sample the sedimentary carapace and upper few meters of basement, to test the hypothesis that the upper surface coincides with a detachment fault, and to help constrain the temporal history of denudation. Collected sedimentary deposits may provide constraints on the timing of exposure across the dome based on the age and isotopic character of preserved microfossils. Basement rock sampled in these short holes provides initial information on deformation and alteration within the exposed fault and, perhaps, rock adjacent to the fault zone.

Site selection was based on a combination of geological and geophysical data (Figs. F3,F4), balancing the details of seafloor character with larger scale objectives attainable if deep penetration of the footwall was successful. Centered within the gently sloping, morphologically corrugated, striated dome (Cann et al., 1997), the site coincides with gravity anomaly and seismic velocity maxima that indicate unaltered ultramafic rocks are likely to be present within several hundred meters of the seafloor (Blackman et al., 1998; Collins et al., 1998, 2001). Argo II imagery and Alvin dive mapping previously showed that the seafloor is covered by a thin layer of unconsolidated sediment deposited on bedrock and interrupted in places by lineated rubble fields (Blackman et al., 2004). In areas without significant loose sedimentary cover, a thin cover of lithified carbonate caps the underlying low-relief basement. Dredge and Alvin sampling (Fig. F2) indicate that loose, angular fragments on the central dome include low-grade metabasalt and serpentinite (Blackman et al., 1998, 2004).

Site U1309 comprises eight holes drilled within 2 km of one another along a spreading-parallel corridor (Fig. F2). The first five holes, Holes U1309A–U1309E, are located within 30 m of each other in an area with 2–4 m of unconsolidated sedimentary deposits above basement. A 60 m × 50 m survey with the vibration-isolated television camera on the drill string documented a ~2000 m2 area where both a single-bit pilot hole and a deep-penetration hole could be initiated within an area free of cobble- to boulder-sized rubble. The area is ~280 m south of an Argo II track (run 039) and an Alvin dive (3642), both from cruise AT3-60 (Blackman et al., 2004), just south of NOBEL Line 10 and west of EW0102 multichannel seismic (MCS) Line Meg-4 at common midpoint 4100 (Canales et al., 2004) (Fig. F3). Towed ocean-bottom instrument (TOBI) and DSL120 side-scan sonar data show spreading-parallel striations crossing this area. A gentle northeast slope coincides with the northern flank of the corrugation that the site penetrates. Principal geologic results from the pilot and deep-penetration holes are presented in subsequent sections.

The series of shallow-penetration holes included in this footwall site are located adjacent to, 280 m northwest of, and 1.6 km northeast of the deep hole (Hole U1309D). The motivation for this series of holes was twofold: first, to check for possible fossils or isotopic signatures in the sedimentary deposits to constrain the exposure age of the hypothesized detachment fault and, second, to attempt recovery of possible fault rock at the top of the domal surface. The first shallow-penetration core was obtained in Hole U1309A. Overcoring in the top interval of the deep-penetration holes (Holes U1309B and U1309D) precluded meaningful recovery of the sedimentary deposits and the upper 20 m of basement immediately below. Additional shallow-penetration holes were drilled in an effort to achieve that goal (Holes U1309E–U1309H).

Hole U1309B

In an effort to assess drilling conditions and begin geologic characterization of the expected detachment fault zone, a single-bit pilot hole was drilled at Site U1309. Hole U1309B, initiated at the same location as Hole U1309A, was cored to 101.8 meters below seafloor (mbsf). Recovery was good (overall average = 46%), increasing significantly (to an average of 52% for 30–100 mbsf) below the upper, very slow drilling 25 m. The hole deviates from vertical by 7° toward the northeast, and this was probably a factor in the reduced quality for some of the downhole logging measurements (see “Downhole measurements”).

Hole U1309C

The attempt to set casing for the hard rock reentry system (HRRS) was not successful in Hole U1309C (30°10.11′N, 42°7.12′W; 1638 mbsl). The hole was abandoned with ~25 m of 13⅜
inch casing pipe standing above the seafloor; it is 20 m east of Hole U1309B and ~30 m southwest of Hole U1309D.

Hole U1309D

Drilling in Hole U1309D (30°10.12′N, 42°07.11′W; 1645 mbsl) took place over four periods for a total of 15 days during IODP Expedition 304 and 34 days during IODP Expedition 305. The hole was spudded using a hammer drill with 13⅜
inch casing in an effort to provide stable reentry for a deep hole. No rock was recovered in the upper 20.5 m of the hole. Below 20.5 mbsf, coring was accomplished using a rotary core barrel (RCB) bit. Despite rough sea conditions during some of the drilling, recovery rates were generally very good. The section from 108 to 126 mbsf (Cores 304-U1309D-18R through 21R) had very low recovery, including one empty core barrel and a second with only 14% recovery. Logging data suggest this low-recovery zone may coincide with a fault zone. Aside from the low-recovery interval and the upper 20.5 m, recovery rates averaged 68% in the upper 400 mbsf (Expedition 304). As drilling conditions in the pilot hole (Hole U1309B) were very good, casing below 20 m in Hole U1309D was deemed unnecessary. This gave us time to core the upper ~130 m during our initial occupation of the hole, providing an opportunity to assess cross-hole correlation of lithologic units and structure between Holes U1309B and U1309D. The second period of coring followed drilling at IODP Sites U1310 and U1311 and deepened the hole to 401.3 mbsf before the first logging run in Hole U1309D completed the work of Expedition 304. Before coring began during Expedition 305, a water sample was obtained from the bottom of the hole (see “Appendix B”). After coring to a depth of ~840 mbsf during Expedition 305, a second logging run was completed. Final coring penetrated to 1415.5 mbsf and was followed by the third set of downhole logging measurements at the end of Expedition 305. Total recovery in Hole U1309D is 1043.3 m (average = 75% below the upper ~20 m).

Hole U1309E

Hole U1309E (30°10.12′N, 42°07.11′W; 1645 mbsl) was offset 10 m east of Hole U1309D for an attempt to recover the sediment and upper meter of basement using the RCB. Disrupted sediments were obtained, as were several fragments of metabasalt.

Hole U1309F

The next attempt at shallow penetration was made in Hole U1309F (30°10.20′N, 42°07.25′W; 1645 mbsl), ~280 m to the northwest of Hole U1309D in an area where unconsolidated sedimentary deposits are less widespread than in Holes U1309A–U1309E and lithified carbonate cap rock was mapped with the Alvin and the Argo II during cruise AT3-60 in 2000. A brief camera survey confirmed the basic setting, although some loose sediment occurred within a few meters of the first contact of the drill bit at the seafloor. Despite clear indications that we drilled >1 m into hard rock, recovery included only disrupted sediment and a few fragments of metabasalt. No chips of lithified carbonate were recognized. We discontinued RCB shallow-penetration attempts and switched to the extended core barrel (XCB) bit for a further attempt (Hole U1309G).

Hole U1309G

Hole U1309G (30°10.54′N, 42°06.32′W; 1872 mbsl) was sited 1.6 km northeast of Hole U1309D in an area characterized by a broad region of variably lithified carbonate deposits above basement. A brief camera survey confirmed this assessment, and the hole was located within sight of a marker left by the Alvin in 2000. The hole was spudded into stepped and platy lithified carbonate sediment. Coring to 3.5 m using an XCB bit recovered 0.91 m of microfossil ooze with three thin (2–3 cm) interlayers of basaltic hyaloclastite. Glass from the hyaloclastite is oxidized palagonite. No lithified carbonate or intact basement rock was recovered. The sequence of fossiliferous ooze, hyaloclastite, and a clayey material with rounded, largely metabasalt clasts may provide useful postexposure data. The latter could be a sedimentary conglomerate, but we cannot rule out significant reworking due to drilling in this lowermost interval.

Hole U1309H

A second attempt at basement recovery from a shallow-penetration hole at the same location as Hole U1309G was possible when logging activities in Hole U1309D were stopped early because of logging tools sticking in the borehole. Rather than risk either tools or hole, the remaining time on site was used to RCB core a few meters at essentially the same location as Hole U1309G. This eliminated the need for a camera survey. Drilling for 4 h penetrated to 4 mbsf; recovery from this hole was 0.19 m and included pieces of basalt and talc-tremolite schist along with one piece of diabase cataclasite (Fig. F5). Despite the small return, these samples are significant. The talc-tremolite schist is similar to fault rocks recovered near the top of the Southern Ridge at Atlantis Massif (Schroeder and John, 2004) and at 15°45′N on the MAR (Escartin et al., 2003). Fracture intensity in the diabase is minor, suggesting fairly low strain, but consistent with the sample being part of a process zone associated with a fault system. Although these samples are minimal, they provide direct evidence that the corrugated central dome of Atlantis Massif is an exposed detachment, consistent with the 50–100 m thick brittle deformation zone described along the top of the south face of the massif (Schroeder and John, 2004; Karson et al., 2005). An Alvin sample within this spreading-parallel corridor on the central dome showed similar talc rock (Blackman et al., 2004) and there were a few chips of talc-tremolite schist in the top core from Hole U1309B, but neither of these latter samples could be proven to be in place.

Igneous sequence petrology and geochemistry

A total of 770 igneous units were defined in Hole U1309D during Expeditions 304 and 305. Each unit is distinguished on the basis of primary modal mineralogy, igneous contacts, and variations in grain size.

The most abundant rock type is from the gabbro group (Fig. F6), comprising 55.7% of the core recovered from Hole U1309D. This group spans a wide range in modal composition, including minor (rarely exceeding a few percent) amounts of olivine, Fe-Ti oxides, and/or orthopyroxene. Gabbroic rocks from Hole U1309D exhibit significant variations in grain size from microgabbro (crystals <1 mm) to seriate medium-grained gabbro to pegmatitic gabbro (grain size >10 cm), in places within a single section (i.e., ~150 cm) of core. Gabbronorite and orthopyroxene-bearing gabbro are included in this group. Because unambiguous identification of orthopyroxene requires careful thin section observation, the amount of orthopyroxene-bearing gabbro is a minimum estimate (see “Igneous petrology”). Gabbronorites show the same textural relationships as the associated gabbros. In the lower part of Hole U1309D (Cores 305-U1309D-243R through 272R), low-Ca pyroxene appears as orthopyroxene and/or inverted pigeonite.

Olivine gabbro is the second most abundant rock type recovered from Hole U1309D (25.5%), with modal olivine varying widely (>5%). The modal composition of this rock type is highly variable on a submeter scale and locally grades into troctolitic gabbro.

Troctolite is commonly spatially associated with olivine and troctolitic gabbros, but it is less common, constituting only 2.7% of the rocks recovered (Figs. F6,F7). The texture of troctolite is irregularly seriate, locally with poikilitic clinopyroxene. Troctolite units are commonly intruded by late-stage dikes of both coarse-grained gabbro and microgabbro.

Olivine-rich rocks with relatively low modal plagioclase and clinopyroxene, including dunite, wehrlite, and troctolite, are grouped as olivine-rich troctolite. They represent 5.4% of the recovered rocks in Hole U1309D, with the thickest interval between 1092 and 1236 mbsf (Fig. F6). The olivine-rich troctolites contain >70% olivine by mode and are commonly intercalated with olivine and troctolitic gabbro (Fig. F8). In contrast to troctolite, olivine-rich troctolite displays subhedral to rounded medium-grained olivine and interstitial to poikilitic plagioclase and clinopyroxene in variable proportions (Fig. F9).

Two short intervals of ultramafic rock were recovered in the upper 100 m of both Holes U1309B and U1309D (Fig. F8C). Four intervals of serpentinized peridotite (three of which are clearly in place) at ~61, 132.5, 172–173, and 224 mbsf were recovered in Hole U1309D and include both lherzolite and dunite. Peridotites from Hole U1309B (~58 mbsf) have Mg# of 90–91. The low CaO and Al2O3 contents of the peridotites suggest that, prior to alteration, they were more refractory than those collected during Ocean Drilling Program (ODP) Leg 153 at 23°N on the MAR (Casey, 1997).

Where visible, contact relations between gabbro and other rock types (except diabase) at Site U1309 suggest that gabbro is generally intrusive into more olivine rich rock types (olivine gabbro and troctolite) and that it is in turn intruded by felsic (“leucocratic”) dikes and oxide gabbro. These relationships are more common between 400 and 650 mbsf than in the lower part of the hole, where gabbro contacts are commonly more diffuse. Contact relations between troctolite and gabbro range from sharp, where thin intervals of gabbro crosscut the serpentine foliation in troctolite, to gradational, where thicker intervals of gabbro are present. Contacts between oxide-bearing rock types and gabbro range from sharp to gradational over centimeters. Felsic dikes in gabbro generally show sharp contacts, but varying degrees of high-temperature reaction between dikes and wallrocks are also observed. These reaction zones commonly have oriented minerals such as oxides, pyroxenes, or plagioclase growing along and across them, suggesting postemplacement reaction.

Holes U1309B and U1309D have interfingered units that vary in thickness from centimeters up to ~100–200 m where intrusive contacts are preserved. The ~140 m thick interval of olivine-rich troctolite from 1094 to 1236 mbsf in Hole U1309D forms an integral lithologic package. The olivine-rich troctolites and minor associated lithologic units that this package comprises have been intruded by numerous crosscutting gabbroic dikes of variable thickness at temperatures below the troctolite solidus. Broader scale contacts with adjacent olivine gabbro appear to be dominantly intrusive and formed under hypersolidus conditions. Interstitial clinopyroxene appears more abundant adjacent to gabbroic dikes, suggesting that oikocrystic clinopyroxene recrystallized during intrusion of the crosscutting dikes.

Diabase intrudes other rock types in several places throughout Holes U1309B and U1309D (Fig. F6C,F6D). The intrusive contacts in Hole U1309B and the upper 130 m of Hole U1309D, taken with the relative intensity of alteration and vein development, suggest that the diabase bodies were emplaced late in the intrusive history of the footwall at Site U1309. Subhorizontal magmatic foliations, together with paleomagnetic and logging data, suggest that diabase in Holes U1309B and U1309D forms groups of subhorizontal sheets or sills. Unit boundaries are locally marked by chilled margins, and in some cases magnetic susceptibility (MS) increases systematically toward the top and base of a unit (Fig. F11). Together, these observations suggest individual diabase sheet thicknesses on the order of 2–8 m.

Nearly all gabbroic rock types recovered from Site U1309 are cut by veins/​dikes (Fig. F12) of variable thickness and composition. These veins and/or dikes range in composition between gabbro, oxide-bearing gabbro, and trondhjemite and may be partly magmatic, partly metamorphic, partly deformation-related, or the result of a combination of all three processes.

Among Deep Sea Drilling Project (DSDP)/​ODP/​IODP drill holes, Hole U1309D is unique in that it represents a section of primitive to somewhat evolved gabbroic rocks and includes intrusive diabase intervals as well as olivine-rich rocks, which may represent primitive cumulates. Gabbroic rocks from Site U1309 have compositions that are among the most primitive sampled by drilling along the MAR (23°N and 15°20′N; Agar et al., 1997; Kelemen, Kikawa, Miller, et al., 2004) and on the Southwest Indian Ridge (SWIR; Hole 735B) (Dick, Natland, Miller, et al., 1999) (Fig. F13). This is reflected in Mg numbers ranging from 67 to 87 (Fig. F14) and low TiO2 (<0.72 wt%), Na2O, and trace element contents. Site U1309 gabbroic rocks can be interpreted as cumulates related to the basalt and diabase through crystal fractionation processes and a common parental magma.

Basalt and diabase from Site U1309 are tholeiitic basalts and minor basaltic andesite, with compositions that overlap basaltic glasses from the entire MAR (Fig. F15). All samples analyzed are slightly CaO and Al2O3 poor and Na2O rich compared to average MAR basaltic glass compositions. These differences may be related to the pervasive greenschist-facies alteration. Samples from the thin diabase units below 130 mbsf are somewhat less evolved in composition than those in the uppermost section. All diabasic rocks show significant variation in incompatible trace elements, including Y and Zr.

Hydrothermal alteration, metamorphism, and metasomatism

Alteration mineral assemblages in rocks from Site U1309 record cooling of mafic plutonic rocks from submagmatic conditions (>1000°C) to the temperatures of zeolite facies (<200°C) during the unroofing and uplift of Atlantis Massif. Individual samples generally display a range of superposed metamorphic conditions, but no single sample records the entire cooling history of the site. Alteration intensity is moderate, tends to decrease downcore, and is commonly related to the intensity of veining (Fig. F16). Locally, there are exceptions to the decreasing alteration downhole where, for example, alteration intensity broadly correlates with the modal abundance of olivine in the intercalated olivine-rich troctolite, olivine gabbro, and gabbro recovered between ~1090 and ~1240 mbsf and in the lowermost gabbros and olivine gabbros. Coarser grained gabbro intervals are, in general, more altered than medium- to coarse-grained units. Intervals of olivine-rich troctolite show alteration restricted to heterogeneous serpentine networks, with strong alteration gradients from the contact with intensely veined intercalated gabbros to the fresher cores of the olivine-rich troctolite units (Fig. F16C). The latter locally contain intervals of very fresh (as low as 1% serpentinization) olivine-rich (as much as ~90%) rocks.

The metamorphic and alteration history recorded at Site U1309 is summarized as follows:

Replacement of pyroxene by green to brown hornblende in diabase, gabbro (especially oxide gabbro), and mylonite zones. The extent of this largely static event is difficult to estimate because of a greenschist-facies overprint by amphibole and uncertainties over amphibole compositions in thin section.

A widespread, largely static upper-greenschist-facies to lower-amphibolite-facies metamorphism manifested by the following:

Formation of secondary plagioclase and amphibole and, below 384 mbsf, epidote growth that appears to be related to late magmatic leucocratic intrusions.

Replacement of pyroxene by actinolitic amphibole. This is the major effect of the greenschist event in most gabbroic rocks and diabase. In the upper ~300 m of Hole U1309D, this alteration is pervasive and all samples are affected to some extent. At greater depths, the alteration is increasingly associated with the emplacement of amphibole-rich veins and accompanying halo alteration (Fig. F18).

Development of tremolite-chlorite ± talc corona texture in all rocks containing both olivine and plagioclase (Fig. F19). This may include some amphibolite-facies formation of cummingtonite and green hornblende. In the upper 300 m of Hole U1309D, this reaction proceeded to completion in almost all samples, removing either olivine or plagioclase from the assemblage. At greater depths, the reaction commonly did not proceed to completion and is increasingly localized by amphibole veins and the margins of gabbroic dikelets. At shallower levels, most amphibole veins postdate corona formation.

Static, lower-greenschist to subgreenschist metamorphism that includes the following:

Serpentinization in olivine gabbro, troctolite, and olivine-rich troctolite with concomitant formation of prehnite and hydrogrossular in associated plagioclase. Above ~300 mbsf, serpentinization is restricted to rocks where olivine was in excess over plagioclase and was therefore still present after the corona-forming reaction went to completion. At deeper levels, serpentine, prehnite, and hydrogrossular are often localized on closely spaced, variably oriented fractures (“ladder veins”). The degree of serpentinization varies widely from >90% to <10% of the olivine. In the simplest case, serpentinization proceeds via the development of kernel texture (O’Hanley, 1996) (Fig. F21). A commonly observed feature in serpentinized rocks is the development of microfracture sets that radiate or extend into plagioclase from neighboring serpentinized olivine grains (Fig. F22). These fractures are commonly filled with chlorite and/or amphibole.

Sporadic talc-carbonate metasomatic alteration of olivine-rich rocks.

Relatively late emplacement of slip-fiber amphibole veins and associated local metasomatism.

Serpentinization of isolated grains in olivine gabbro and relict grains in coronas.

Zeolite-facies metamorphism that includes replacement of plagioclase by zeolites throughout the core and emplacement of zeolite-bearing veins below 700 mbsf (Fig. F23,F24). Late, open, irregular fractures commonly contain a clay mineral that may be saponite, along with carbonate and (below 700 mbsf) zeolite minerals.

Structural relationships

The majority of core recovered from Site U1309 records pervasive static alteration of the rocks and shows that pseudomorphs of igneous textures remain largely unmodified. Magmatic deformation fabrics (defined by the preferred orientation of plagioclase) were recorded in 22% of recovered rocks (Fig. F24). These fabrics are weak except in local intervals. Magmatic foliation tends to be better developed in finer grained gabbros than in coarser grained varieties; foliation is also well developed in the rare layered intervals. Textural observations suggest that some foliation may have been destroyed by late growth of pyroxene crystals reaching sizes as large as 20 cm. Magmatic foliations typically dip ~30°–60° but are steeper (e.g., at 400 and 560 mbsf) or more gently dipping (e.g., at 850 and 1150 mbsf) in local intervals. In many places, weak to moderate crystal-plastic deformation seems to overprint magmatic foliations (Fig. F25), as also described in Hole 735B (Shipboard Scientific Party, 1999). This deformation is commonly difficult to identify macroscopically but is evident in thin section.

High-strain crystal-plastic shear zones are rare (recorded in only 3% of the core). The highest densities of crystal-plastic shear zones are at 35–80 mbsf and 670–720 mbsf in Hole U1309D and are typically restricted to clearly defined, mostly granulite grade shear zones ranging in width from millimeters to a maximum of a few meters. Both normal and reverse senses of offset in the core reference frame are observed in such shear zones, and their dips are typically moderate but can be locally steep (e.g., at 700 mbsf). This is in marked contrast to the much larger number of high-strain shear zones recorded in Hole 735B on the SWIR, especially in the upper 500 m (Dick, Natland, Miller, et al., 1999; Dick et al., 2000).

Vein sets defined by mineralogy are found throughout the core. The earliest generation is late magmatic veins. Their occurrence broadly correlates with country rocks with a similar composition, suggesting local derivation (scale = ~100 m). Later generations of veins include dark green amphibolite-facies veins (consisting of amphibole) cut by pale green, fibrous greenschist-facies veins (actinolite/tremolite-chlorite ± talc and epidote). Fiber orientations on the pale green veins are commonly subhorizontal (particularly deeper than ~300 mbsf) independent of vein-dip, indicating strike-slip movement during formation. The latest (and lowest metamorphic grade) vein type is typically open, white veins (carbonate and sulfide ± chlorite; prehnite), possibly associated with unloading. Gray veins (serpentine and chlorite) are spatially restricted to olivine-rich troctolite and both cut and are cut by serpentine foliation. In hand sample, such areas of intense serpentinization form a conspicuous but generally irregular foliation (Fig. F26) with orientations varying on a meter scale or from one piece of core to the next, and several cross-cutting foliations developed in the same piece of core. Subsets of the gray veins have synkinematic fibers. Vein intensities tend to correlate with fault zones on a local scale (Fig. F27), though displacement on veins (vein faults) is not common. Vein intensities decrease significantly below 785 mbsf. The dip of veins is variable but moderate on average, irrespective of the vein type. On the scale of the entire core, there is no systematic, lithology-dependent downhole distribution pattern of specific vein types.

The amount of strain recorded by brittle fracture and cataclasis is negligible overall, except for fault zones concentrated in the upper 50 m of Hole U1309D, at 108–126 mbsf and between 685 and 785 mbsf (Fig. F28), where the boundaries of structural units are defined (Fig. F27). Additional zones of significant cataclasite were found at 250 and 1100 mbsf. Cataclasis is locally associated with oxide gabbro intervals/​dikelets, leucocratic veins, and contact zones between diabase intrusions and their gabbroic host rocks. Crosscutting relationships indicate a complex succession of events involving fluid flow and deformation.

Structurally, core from Hole U1309D can be subdivided into three major units (Fig. F27):

Structural Unit 1 (0–170 mbsf) is marked by a high but decreasing degree of cataclasis downhole; abundant, late, relatively undeformed diabase; a high degree of greenschist-grade alteration; and a near-present-day orientation of the paleomagnetic inclination. The boundary to structural Unit 2 at ~170 mbsf is marked by a subhorizontal to moderately dipping crystal-plastic shear zone within gabbroic rocks, a high intensity of veining, strong cataclasis, and a ~2 m thick interval of altered ultramafic rocks.

Structural Unit 2 extends from ~170 to ~785 mbsf. It is marked by a relatively high intensity of veining, including the presence of sulfides. Paleomagnetic inclinations are ~10°–30° shallower than present-day values. Lithologically, structural Unit 2 is varied and nondistinct. The base of structural Unit 2 is defined by a series of greenschist-grade cataclastic fault zones occurring between 695 and 785 mbsf. There is a sharp decrease in whole rock Mg# of the gabbros at ~600 mbsf (Fig. F14).

Structural Unit 3 extends from this boundary to the bottom of Hole U1309D and is characterized by an overall low intensity of cataclastic deformation, veining, and plastic deformation.

Tentative reorientations of structures using paleomagnetic and logging data were performed on structural features from Holes U1309B and U1309D to ~130 mbsf. These data indicate that crystal-plastic foliations dip dominantly to the west, a majority of veins dip toward the east, and several faults strike east-west. Holes U1309B and U1309D are 20 m apart, and thus the local continuity of igneous, metamorphic, and structural units can be evaluated. On a broad scale, lithologic correlations between Holes U1309B and U1309D are possible, but correlation breaks down at a scale of <10 m (see “Structural geology”).

The lack of significant structures indicative of high displacement by either ductile or brittle processes severely limits the possible thickness of fault zones that could comprise a detachment system over the central dome. Poor recovery of the upper 20 m of the footwall allows the possibility that this narrow zone accommodated very high strain along a brittle fault. Such extreme strain localization has been documented to occur associated with continental detachments (e.g., John, 1987; Miller, 1996). If this is the case here, the central dome differs from the southern ridge of Atlantis Massif, where seafloor mapping and sample analysis suggest a detachment zone thickness on the order of 50–100 m (Schroeder and John, 2004; Karson, 2003).

Geophysical measurements

Shipboard physical property data in combination with downhole logging data provide an initial means to assess which aspects of the geological characteristics of the domal core of Atlantis Massif might contribute to the regional geophysical data sets. In addition, inherent rock properties can be assessed and related to rock type and alteration.

MS is highest in the olivine-rich troctolites recovered from Hole U1309D but is quite low in most of the gabbros (Fig. F29). Both olivine-rich troctolite and oxide gabbro intervals can have very high MS signals (2,000–10,000 instrument units [IU]), although only the former is consistently at these levels. The olivine-rich troctolites are variously serpentinized, with very strong local gradients, and the susceptibility reflects magnetite produced during the alteration process.

Natural remanent magnetization (NRM) of the rocks from Hole U1309D was determined on board following removal of the drilling-induced overprint. Alternating-field demagnetization (typically 30 mT) was used to remove the overprint, and the bulk of the archive-half sections show negative inclination direction (Fig. F27C), which corresponds to a reversed magnetic polarity epoch. Minicore samples, cleaned of overprint by either alternating-field (up to 100 mT) or thermal demagnetization (to 500°–550°C), generally show very good agreement with the half-core inclination patterns downhole. Five inclination groups have been identified based on remanence data from archive halves and discrete samples. The boundaries between these groups generally coincide with structural features (faults and shear zones), so structural data have been used to define the precise boundaries.

The upper 180 m is characterized by a mean discrete sample inclination (–49°) that is identical to the expected dipole inclination at the site (Fig. F27C). This suggests that from 0 to 180 mbsf there has not been detectable tectonic rotation since the remanence was acquired, although it should be kept in mind that modest but significant (~20°) counterclockwise rotation about a 10° trending, horizontal axis (i.e., subparallel to the ridge axis) would produce no net change in the inclination of the remanent vector. In contrast, the mean inclinations from all inclination groups deeper than 180 mbsf in the hole are statistically distinct from the expected direction (Fig. F27C). The shallower inclinations in these sections cannot be attributed to artifacts of the drilling and/or measurement process, as they are corroborated by numerous high-quality discrete sample demagnetization data. Inclination Groups III and IV have mean values steeper than those from Groups II and V. This nonsystematic change in remanence inclination downhole is difficult to reconcile with any simple model of denudation. Instead of the footwall behaving as a single block, it appears that different intervals (inclination Groups I–V) may have experienced different tectonic and/or alteration histories. Because structural boundaries generally coincide with the inclination group boundaries (Fig. F27), those deformation zones could provide a mechanism for variation in tectonic rotation within the footwall.

Multicomponent remanences were documented for a few samples in the upper 400 m of Hole U1309D, and this suggests that remanence acquisition spanned multiple polarity intervals. The highest stability reversed polarity magnetizations are typically shallower than the normal polarity overprint (Fig. F30), and the two components are not antipodal. These overprints, distinct from the steep, low-coercivity drilling-induced component, have a mean inclination (53.6° +6.4°/–8.2°) that is not statistically distinguishable from the present-day normal polarity inclination at the site. The difference between the normal and reversed polarity directions may reflect the influence of tectonic tilting after acquisition of the highest stability reversed polarity magnetization. Although less common, such multicomponent magnetizations are also observed in several discrete samples analyzed from the lower 1000 m of the hole.

The physical properties that are most relevant for relating the rocks from Hole U1309D to broader scale geophysical measurements are seismic velocity and density. Shipboard measurements provide an indication of the inherent properties of small samples at room temperature and pressure. Variability in the measured values can be due to a number of factors, including mineralogy, porosity, grain size, and the style and degree of alteration.

A significant change in several core sample and downhole logging properties occurs between ~280 and ~350 mbsf, across an interval dominated by serpentinized, olivine-rich rocks (Fig. F31). Density values have reduced scatter and slightly higher average values below 350 mbsf, increasing from 2.8 g/cm3 in the interval 280–340 mbsf to 2.9 g/cm3 in the interval 350–400 mbsf. Average compressional velocity of minicore samples in the 280–340 mbsf interval drops to 5.3 km/s (from 5.5 km/s in the overlying 200 m) before increasing to 5.7 km/s at 340–400 mbsf. Logging compressional velocity increases from ~5.5 to 6.0 km/s between 340 and 370 mbsf. These changes combine to produce an impedance contrast (Fig. F32), and this can be related to the seismic reflection data. Electrical resistivity measured by the Dual Laterolog shows a marked decrease in the ~280–340 mbsf interval and then increases significantly below 350 mbsf in the underlying gabbroic interval (Fig. F31). Overall alteration in the serpentinized, olivine-rich interval is greater than in the overlying section (average = 50%–75%), and it drops steadily to 20%–40% by 400 mbsf. Higher velocity and density values correspond to the underlying gabbroic interval.

A second strong gradient in borehole electrical resistivity is present from 730 to 760 mbsf. Core sample porosity drops from an average of 1.5% (400–700 mbsf) to 1% (700–800 mbsf) and maintains an average of 0.6% at greater depths. These physical property changes approximately coincide with a rapid drop in overall level of alteration (Figs. F16B,F33) across this interval. Intriguingly, below ~925 mbsf, average sample velocities steadily decrease from ~5.9 to ~5.5 km/s at 1415 mbsf, whereas the densities remain stable on average (Fig. F31). The cause of this decrease in compressional velocity is not clear but could be related to microcracking due to unloading during the coring process at these depths.

The check shot experiment extended from 275 to 840 mbsf with average station spacing of ~50 m. Tool failure and high seas combined to preclude collection of any seismic measurements during the final logging run. The check shot data indicate an average velocity in the upper 550 m of the footwall in Hole U1309D of 5.5–5.6 km/s. An increase is indicated for greater depths by higher average velocities (5.8 km/s for stations 580–796 mbsf). Interval velocities computed from the automatic picks on stacked seismograms vary between stations. At this stage, we cannot rule out the determination of a velocity of >7 km/s between 580 and 635 mbsf. Adjustments that could compensate for an error in seismometer position(s) of several meters do not change the basic result of a thin higher-than-average velocity interval. It is possible that a lens of essentially unaltered, olivine-rich rock occurs within the Fresnel zone of the check shot experiment at Hole U1309D (on the order of a few hundred meters), although confidence in this result will require additional seismic measurements.

Temperature in the borehole increases with depth as expected (Fig. F34). Because of the significant impact of drilling in the hole, the measurements made provide minimum estimates of what the actual formation temperature is. The Temperature/​Acceleration/​Pressure (TAP) tool recorded a temperature of 120°C at the bottom of the hole (1415 mbsf). The temperature is somewhat lower than predictions from a simple cooling plate model of a spreading ridge flank with an age of ~2 Ma. However, the measured temperatures are a minimum (owing to hole cooling during coring), and much more careful measurement is required before quantitative interpretation should be made. These initial results suggest that thermally driven flow in the hole is likely to occur. Several drops of a few degrees were recorded in narrow intervals on repeated TAP runs (Fig. F34). At least two of these coincide with documented fault zones (~785 and 1107 mbsf), perhaps indicating fluid flow there.

Microbiology

A total of 12 samples for microbiological investigations were collected from whole-round core samples recovered from Holes U1309B and U1309D during Expedition 304. Sample depths ranged from 0.45 to 396.5 mbsf, and all major rock types were included: carbonate sediment, basalt, diabase, gabbro, and serpentinized peridotite. Shipboard cultivation studies indicate growth of matter from two altered gabbro samples at elevated temperature, based on positive fluorescence tests. Shore-based analyses are required to confirm that this is a microbial signature as opposed to being due to inorganic material.

Additional microbiological studies were conducted on 15 whole-round core samples taken from 401 to 1391 mbsf during Expedition 305. Gabbro, olivine gabbro, and olivine-rich troctolite were sampled. In an effort to establish a culture collection of endolithic microbes, four different types of media, as well as agar plates, were inoculated with portions of these core samples. Growth was observed from two samples based on a positive fluorescence test. Again, because of the autofluorescence of rock particles, the presence of microorganisms can neither be confirmed nor denied until a shore-based molecular analysis of cultures is undertaken.